Existing methods of creating electrical and mechanical contacts between conductive pads include soldering, using anisotropic conductive paste (“ACP”) or film (“ACF”), and conductive adhesives.
Soldering is commonly used in the electronics industry. However, in order to create a good electric contact both pads are heated to relatively high temperatures. This method is unsuitable when the electric pads are part of polymeric substrates, such as a PET foil, due to the relatively high temperatures required to practice this option.
Anisotropic conductive materials, such as ACF or ACP, are common alternatives for soldering. ACP/ACF works by trapping conductive particles between the corresponding conductive pads. An exemplary prior art method of using an anisotropic material is shown in FIGS. 1A and 1B. Referring to FIG. 1A, a pre-bonding step is shown. During the pre-bonding step, an ACF or ACP 102 is placed between a PET foil 104 and a printed circuit board (“PCB”) substrate 106. The ACF/ACP 102 consists of conductive particles 108 dispersed within a non-conductive epoxy/adhesive. The electric pads 110 on PCB substrate 106 and PET foil 104 are slightly elevated from the surface. The bonding step consists of applying pressure and heat across the top surface of the PCB substrate 106. As seen in FIG. 1B, after the bonding process, some of the conductive particles are trapped between the electric pads 110 of PCB substrate 106 and the electric pads 114 of the PET foil 104. The trapped particles form an electric contact in the vertical direction. The conductive particles in the medium 112 between the electric pads are distributed in a way that doesn't allow electrical contact in the lateral direction. After the bonding process, the epoxy material 102 is cured and the conductive particles are locked in their compressed position. The curing technique depends on the anisotropic conductive material.
In order to ensure sound electrical contacts, the bonding process must provide an even amount of heat and pressure across the entire surface of the top substrate 106. These requirements lead to some of the ACF/ACP major disadvantages including:                1. the PCB substrate 106 has to be of limited dimensions in order to ensure an even distribution of pressure and heat;        2. the top surface of the PCB substrate 106, opposite the electric pads 110, has to be smooth;        3. at least one of the substrates 104, 106 has to be thin and flexible;        4. high stress levels do not allow assembling of brittle parts; and,        5. the typical curing temperature of the ACF/ACP 102 is greater than 120° C., which is still relatively high.        
These disadvantages make it impossible to use ACF/ACP on uneven surfaces such as rigid PCB boards assembled with electronic components because standard bonding machinery supports limited surface areas and specific geometries. In addition the ACF/ACP will not allow assembly at low temperature.
A third alternative for creating electrical and mechanical contacts between electric pads includes using a conductive adhesive. However, the use of a conductive adhesive, such as Epo-Tek E4110, is very likely to result in lateral conductance. Lateral conductance is undesirable because it causes a short circuit between the electric pads. FIG. 2 shows an attempt to use a conductive adhesive 202 in order to electrically connect the electric pads 204 on a first substrate 206 to the electric pads 208 on a second substrate 210. Applying even a small amount of pressure to the first substrate 206 will cause the adhesive 202 to spread sideways, in which case, undesirable lateral conductance is likely to occur.